1. Field of the Invention
The field of art to which the claimed invention pertains is hydrocarbon separation. More specifically, the invention relates to a process for separating a para-isomer from a feed mixture comprising at least two-bi-alkyl substituted monocyclic aromatic isomers, including the para-isomer, the isomers having from 8 to about 18 carbon atoms per molecule which process employs specially prepared zeolitic adsorbent which selectively removes the para-isomer from the feed.
2. Description of the Prior Art
I have discovered that ion-exchange particles of a base material comprising type X or type Y zeolite and amorphous material as a binder with an aqueous solution of sodium hydroxide prior to the ion exchange with certain selected cations produces an adsorbent possessing faster adsorption-desorption rates when used to separate the para-isomer from a feed mixture comprising at least two bi-alkyl substituted monocyclic aromatic isomers, including the para-isomer, the isomers having from 8 to about 18 carbon atoms per molecule. The reason for this is not entirely understood but it is hypothesized that the ion-exchange of the base material with aqueous sodium hydroxide replaces extraneous non-sodium cations, such as H+ or Group IIA cations, present in the base material and occupying exchangeable sites within the zeolite thereby permitting higher amounts of the selected cations to be added during a subsequent ion-exchange step.
It is known in the separation art that certain adsorbents comprising crystalline aluminosilicates and containing selected cations can be used in processes to separate hydrocarbon isomers from feed mixtures containing such isomers. For example, U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; 3,663,638; and 3,734,974 teach the use of adsorbents comprising X or Y zeolites and selected cations in a process for the separating a specific para-isomer, para-xylene, from a mixture of C.sub.8 aromatic hydrocarbons.
The prior art has also recognized that the certain properties or characteristics of various zeolites can be modified by treating the zeolites with various substances. Typically, zeolites are treated to eliminate or suppress an undesireable characteristic such as acidity which may bring about such acid-catalyzed reactions as isomerization and polymerization. Various zeolites may also be treated to enhance a particularly desired characteristic when employed in specific processes.
U.S. Pat. 3,382,039 for example relates to a process for increasing the exchange capacity of silver zeolites over that of known silver zeolites and their use in providing potable water from saline waters. U.S. Pat. No. 3,326,797, for example, discloses a process for aqueous caustic treating of high silica zeolites having silica over alumina ratios between about 6 and 12, at treating conditions, for the sole purpose of removing a certain percentage of structural silica from the zeolite. The caustic treatment, at conditions to preferably retain a final SiO.sub.2 Al.sub.2 O.sub.3 ratio greater than about 5.5, is found to increase the adsorptive capacity of the zeolite and to increase its catalytic activity when used in hydrocatalytic conversion processes such as olefin hydrogenation, hydrocracking, and desulfurization. The caustic treating process of that reference patent is concerned only with etching or leaching of silica from the zeolite structure to achieve these characteristics.
The prior art references either alone or in combination, however, do not disclose or suggest the adsorbent preparation method of this invention or the aromatic isomer separation process of this invention which process employs the adsorbent so produced.